The present invention relates to a cooling air flow controlling apparatus for a gas turbine, and more particularly, to an apparatus for controlling cooling air at an optimum discharge amount according to a combustion condition of the gas turbine being cooled with the cooling air.
It is well known that gas turbine efficiency can be improved rapidly with higher firing temperature as a working medium. However, with increases in the firing temperature, it is necessary to make improvements in component materials.
To provide an increase of the firing temperature, the use of the high temperature-resisting material is essential. Under existing techniques, however, material development has levelled off and substantial improvements in materials are not expected. Therefore, a method for lowering the working material temperature is achieved frequently by cooling the material itself.
Among the components members of the gas turbine, a first stage stator blade is exposed to the maximum firing temperature, and the temperature condition is mitigated at a first stage rotor blade, a second stage stator blade, and a second rotor blade in order. The gas turbine used as a power generating plant adopts a cooling method which includes the steps of compressing by an air compressor directly coupled with the gas turbine, cooling the compressed air with an intercooler, and cooling the stator blade and the rotor blade by the decreased temperature air.
For the sake of an increase of the power generating plant efficiency, the cooling air consumption is desired to be the minimum required amount so as to maintain the temperature in the stator blade and the rotor blade at an allowable value.
In, for example, Japanese patent laid-open publication No. 112826/1980, is proposed that the exhaust gas temperature of the gas turbine is detected, and a cooling air flow is controlled according to the detected exhaust gas temperature.
In this conventional apparatus, when the atmospheric temperature is constant, there is a correlation between the exhaust gas temperature and the firing temperature, so that the control of the cooling air flow can be done accurately. However, when the atmospheric temperature changes, the above correlation is incorrect and problems result.
Therefore, the atmospheric temperature is set at a maximum temperature, for example 50.degree. C., corresponding to the worst condition expected cooling, and the required cooling air flow determined according to the exhaust gas temperature is correlated by the above atmospheric temperature setting. For that reason, when the actual atmospheric temperature is lower than 50.degree. C., more than the necessary amount of cooling air flow is supplied into the apparatus, thereby the power generating plant efficiency decreases.
It has also been considered that the firing temperature of the gas turbine may be measured directly by means of a sensor, and then the cooling air flow may be controlled according to the measured firing temperature. However, when the firing temperature is above 1000.degree. C. and flows in a gas path at high speed, it is difficult to obtain a sensor having high reliability over a long term, so that this control method has not yet been realized for practical use.